The invention relates to a system for controlling the attitude of a satellite in orbit, which system is more particularly intended to be used when great agility is required of the satellite. It also relates to a satellite control method employing the system.
Operating a satellite in orbit in space requires the satellite to be equipped with a system for controlling its attitude so that it can be oriented in a particular manner and stabilized when it is correctly oriented.
In some forms of operation which require great agility of the satellite, for example a remote sensing satellite, it is important for the attitude control system to be able to change the orientation of the satellite quickly and to stabilize the satellite quickly after such changes.
A satellite can be oriented and stabilized with relatively low expenditure of energy by an attitude control system entailing the satellite reacting to a moving mechanism within the satellite. This is known in the art. The energy employed can be renewable energy.
One example of a control system of the above kind is described in U.S. Pat. No 3,452,848. The system described uses gyroscopic actuators each including a motorized rotor which is used to create angular momentum of constant amplitude and a gimbal system for modifying the orientation of the rotor and consequently its angular momentum. Angular momentum can be exchanged very quickly between a gyroscopic actuator and the satellite carrying it. This is known in the art. However, actuators of this kind are somewhat inaccurate unless the specification is very severe, in particular with regard to the motors. The ratio between the accuracy of the output torque of a gyroscopic actuator and that of its gimbal motor is a function of the angular momentum. Also, a gyroscopic actuator generates a rotating torque, although a fixed torque would be preferable. The guidance law for a satellite equipped with a control system using gyroscopic actuators is therefore generally relatively complex and leaves room for singularities which correspond to situations in which the system is incapable of supplying a torque in a given direction, which can lead to loss of control over the satellite. However, despite those drawbacks, gyroscopic actuators of the above kind are used because they have a high torque capacity, enabling a satellite equipped with them to be rotated quickly, at the cost of a particularly severe specification and uprating of their control system, and even though they are consequently of high cost. This is known in the art.
Reaction wheels have the advantage of being mechanically less complex than gyroscopic actuators but the disadvantage of producing only a relatively low torque, compared to that which can be produced by a gyroscopic actuator. This is also known in the art. A reaction wheel can produce a torque of the order of 1 Nm, for example, compared to a torque of several thousand Nm for a gyroscopic actuator.
The invention therefore proposes a satellite attitude control system.
According to a feature of the invention, the system includes a programmed processor system which includes a gyroscopic actuator first control stage for changing the attitude of the satellite and a reaction wheel second control stage for assuring that pointing of the satellite is accurate and stable.
The invention also proposes an attitude control method for a satellite equipped with a gyroscopic actuator first control stage and a reaction wheel second control stage.
According to a feature of the invention, the method uses the actuator first stage to perform changes of attitude of the satellite on command, and it uses the reaction wheel second stage to point and stabilize the satellite.
According to the invention, the method also uses the reaction wheel second stage to accumulate unwanted torques which affect the satellite.